Glasses with very high index of refraction and low density

ABSTRACT

This invention relates to glasses exhibiting refractive indices higher than 1.880, Abbe numbers of at least 29, densities lower than 4.1, good chemical durability and stability against devitrification, and an integrated transmission over the range of 380-800 nm at a thickness of 10 mm greater than 79%, the glasses consisting essentially, expressed in terms of weight percent on the oxide basis, of 
     
         ______________________________________                                    
 
    
     SiO 2             5-8                                                  
B 2  O 3       15-21                                                
ZrO 2              3-10                                                
TiO 2              7-17                                                
Nb 2  O 5      20.5-26                                              
ZrO 2  + TiO 2  + Nb 2  O 5                                   
                     29-45                                                
La 2  O 3      19-32                                                
Y 2  O 3       0-9                                                  
CaO                   8-16                                                
SrO                  0-5                                                  
BaO                  0-5                                                  
MgO                  0-5                                                  
CaO + SrO + BaO + MgO                                                     
                      8-16                                                
ZnO                  0-5                                                  
Li 2  O           0-2                                                  
Na 2  O           0-2                                                  
K 2  O            0-2                                                  
Li 2  O + Na 2  O + K 2  O                                       
                      0-2.                                                
______________________________________

BACKGROUND OF THE INVENTION

The present invention relates to glass compositions for themanufacturing of lenses for optical and/or ophthalmic use, with an indexof refraction (n_(d)) higher than 1.88, a low dispersion (Abbe numbergreater than or equal to 29), and a density lower than approximately4.g/cm³ .

For spectacle glasses of high correction glasses, the use of a glasswith an index of refraction of approximately 1.9 permits, when comparedto glasses with a lower index of refraction (1.7, for example) and equalpower, appreciable reduction of the thickness at the edge (when it is amatter of negative power lenses) and in the center (when it is a matterof positive power lenses).

This constitutes an undeniable advantage for the wearer of thespectacles, in particular in case of high corrections (>6 diopters). Itis also known that increasing the index of refraction is generallyaccompanied by a deterioration of the other properties characteristic ofthe glass, and in particular the density increases. For a glass withhigh index to be able to retain the advantages of comfort mentionedabove, it is necessary for its density not to be too high. Anotherdisadvantage often associated with increasing the index is the increaseof dispersion (or reduction of the Abbe number).

By reason of the above-mentioned constraints, and yet others regardingtoxicity and cost, it is difficult to find satisfactory glasscompositions with high index of refraction.

SUMMARY OF THE INVENTION

The present inventors have nevertheless succeeded in this and have foundnew glass compositions belonging to the family of the lanthanum borates,which, contrary to the majority of the other known glasses of thisfamily, do not contain heavy metal oxides, such as Ta₂ O₅ or PbO, ortoxic metal oxides, such as ThO₂ or CdO, or very expensive oxides, suchas Gd₂ O₃ or HfO₂.

More precisely, the invention relates to glasses characterized by thefact that they exhibit an index of refraction higher than 1.880, an Abbenumber of at least 29, a density lower than 4.1 g/cm³ a low tendencytowards crystallization, and an integrated transmission from 380 to 800nm--at a thickness of 10 mm--higher than 79%; these glasses having thefollowing basic chemical composition, expressed in weight percent on theoxide basis:

    ______________________________________                                        SiO.sub.2            5-8                                                      B.sub.2 O.sub.3      15-21                                                    ZrO.sub.2             3-10                                                    TiO.sub.2             7-17                                                    Nb.sub.2 O.sub.5     20.5-26                                                  ZrO.sub.2 + TiO.sub.2 + Nb.sub.2 O.sub.5                                                           29-45                                                    La.sub.2 O.sub.3     19-32                                                    Y.sub.2 O.sub.3      0-9                                                      CaO                   8-16                                                    SrO                  0-5                                                      BaO                  0-5                                                      MgO                  0-5                                                      CaO + SrO + BaO + MgO                                                                               8-16                                                    ZnO                  0-5                                                      Li.sub.2 O           0-2                                                      Na.sub.2 O           0-2                                                      K.sub.2 O            0-2                                                      Li.sub.2 O + Na.sub.2 O + K.sub.2 O                                                                 0-2.                                                    ______________________________________                                    

The ranges of proportions mentioned above for the glass constituents arevery important for obtaining the physical and chemical propertiesrequired for the contemplated application.

In particular, the introduction of SiO₂ into the composition allows theviscosity at the liquidus to be increased slightly and the chemicaldurability to be improved. A SiO₂ content higher than 5% is necessary toguarantee a good durability of the glass. The maximum SiO₂ content mustnot exceed 8%, the value beyond which the glass has a tendency tocrystallize.

For a B₂ O₃ content less than 15%, the composition is not very stablewith regard to devitrification, and beyond 21% the high indexes expectedare impossible to attain.

Among the oxides exercising a large contribution to the index ofrefraction, such as ZrO₂, TiO₂, and Nb₂ O₅, a compromise must be foundbetween the amounts of these three elements so as to maintain theclaimed properties.

The presence of ZrO₂ allows the viscosity at the liquidus to beincreased. A ZrO₂ content higher than 3% contributes to the chemicalstability of the glass and improves the durability, but the tendencytowards devitrification is accentuated for contents higher than 10%.ZrO₂ exhibits the disadvantage of increasing the density of the glass.

TiO₂ in contrast allows one to produce the best index-density compromiseto be produced, and a TiO₂ content higher than 7% allows a low densityto be maintained, but, with high contents, the transmission of the glassin the visible light range decreases; a characteristic yellow tint moreor less pronounced as a function of the TiO₂ content is observed, andthe Abbe number decreases rapidly. This is the reason why its contentmust not exceed 17 weight percent.

Nb₂ O₅ also allows one to increase the index of refraction withoutexcessively lowering the constringence; it therefore proves attractive,compared to TiO₂, in spite of its high price. Its content must not,however, exceed 26% in order to avoid devitrification of the glass.

A Nb₂ O₅ content of at least 20.5% must be observed in order to obtaingood durability, as well as a reduced tint and a good level of lighttransmission.

By maintaining the total ZrO₂, TiO₂, and Nb₂ O₅ content between 29 and45 weight percent, it is possible to assure a good compromise oftransmission/durability properties.

La₂ O₃ figures also among the oxides exercising a large contribution tothe index of refraction in like manner to ZrO₂, TiO₂, and Nb₂ O₅considered above. This is the compound, however, which offers the bestindex-dispersion compromise. This is why it must be present in the glassin a proportion of at least 19 weight percent. Its content must notexceed 32 weight percent because beyond this the addition of La₂ O₃deviates disastrously with regard to the density and devitrification ofthe glass.

Y₂ O₃ can optionally be introduced into the glass in order to increasethe constringence and to improve the chemical stability. Because of itsvery high price, however, one prefers not to use it or to use it only ina small quantity in the glass. In any case, its weight content must notexceed 9% under pain of having a glass which is not very stable withregard to devitrification.

CaO, SrO, BaO, MgO, and ZnO are bivalent oxides exercising a moderatecontribution to the index.

CaO, contrary to ZnO, SrO and BaO, allows one to avoid excessivelyincreasing the dispersion and the density. For this reason, its contentmust be kept between 8 and 16 wt %. A CaO content of at least 8% isnecessary to obtain an index of refraction higher than 1.88 with at thesame time the other desired properties, particularly the Abbe number. Itmust not, however, exceed 16% under pain of decreasing the aciddurability and promoting devitrification. SrO, BaO, and MgO are optionaland must not be present in a proportion higher than 5% each. Moreover,when a combination of CaO, SrO, BaO and/or MgO is used, it must observethe following condition: 8≦CaO+SrO+BaO+MgO≦16 weight percent so as toobtain the above-mentioned properties.

Just as for the other divalent oxides except CaO, ZnO is optional, andits content must not exceed 5 weight percent so as not to excessivelyincrease the density.

The oxides of alkali metals Li₂ O, Na₂ O, and K₂ O, with smallcontribution to the density, are optional and are only tolerable insmall proportions because they lead to a rapid reduction of the index ofrefraction and, further, deteriorate the durability in acid medium ofthe glass and promote its devitrification. Thus, their content must bemaintained at less than 2 weight percent.

Besides the ingredients above, the presence of arsenic (As₂ O₃) andnitrates can be desirable for optimizing the state ofoxidation-reduction of the glass and therefore of its tint and of itstransmission, as is well known in the glass making industry in general.

Thus, for this purpose, a certain quantity of oxides can be introducedin the form of nitrates, and a proportion of As₂ O₃ less than 0.5 weightpercent can be introduced as a fining agent in the compositions of theinvention.

In order to obtain glass having an excellent durability in acid medium,that is to say a weight loss in the test DIN 12116 less than 4000mg/dm², the glass constituents will preferably be comprised within theranges specified below:

    ______________________________________                                        SiO.sub.2            6-8                                                      B.sub.2 O.sub.3      17-18                                                    ZrO.sub.2             6-10                                                    TiO.sub.2             7-11                                                    Nb.sub.2 O.sub.5     20.5-26                                                  ZrO.sub.2 + TiO.sub.2 + Nb.sub.2 O.sub.5                                                           34-41                                                    La.sub.2 O.sub.3     19-26                                                    Y.sub.2 O.sub.3      0-8                                                      CaO                  12-16                                                    SrO                  0-5                                                      BaO                  0-5                                                      MgO                  0-5                                                      CaO + SrO + BaO + MgO                                                                              12-16                                                    ZnO                  0-5                                                      Li.sub.2 O           0-2                                                      Na.sub.2 O           0-2                                                      K.sub.2 O            0-2                                                      Li.sub.2 O + Na.sub.2 O + K.sub.2 O                                                                 0-2.                                                    ______________________________________                                    

DESCRIPTION OF PREFERRED EMBODIMENT

The invention is illustrated by the nonlimiting examples given in TableI. The preferred example is No. 5. All the proportions are expressed inweight percentage.

PREPARATION OF THE GLASSES

The various oxides constituting the glass are supplied by the batchmaterials specified below:

    ______________________________________                                        Oxides            Batch materials                                             ______________________________________                                        SiO.sub.2         Quartz                                                      B.sub.2 O.sub.3   B(OH).sub.3                                                 ZrO.sub.2         ZrO.sub.2                                                   TiO.sub.2         TiO.sub.2                                                   Nb.sub.2 O.sub.5  Nb.sub.2 O.sub.5                                            La.sub.2 O.sub.3  La.sub.2 O.sub.5                                            Y.sub.2 O.sub.3   Y.sub.2 O.sub.3                                             CaO               CaCO.sub.3, Ca(NO.sub.3).sub.2                              SrO               SrCO.sub.3                                                  BaO               BaCO.sub.3, Ba(NO.sub.3).sub.2                              MgO               MgCO.sub.3                                                  ZnO               ZnO                                                         Li.sub.2 O        Li.sub.2 CO.sub.3                                           Na.sub.2 O        Na.sub.2 CO.sub.3, NaNO.sub.3                               K.sub.2 O         K.sub.2 CO.sub.3, KNO.sub.3                                 ______________________________________                                    

Preferably, one will choose batch materials so as to exclude as much aspossible the presence of oxides of transition metals, in particular Fe₂O₃, whose total content analyzed is advantageously kept less than 10ppm, in order that the glass obtained exhibits an integratedtransmission within the limits defined above.

After weighing, the various batch materials are mixed according tocurrent techniques. The mixture is then put in a kiln in a platinumcrucible at a temperature of approximately 1200° C. When it iscompletely melted, the temperature of the melt is brought toapproximately 1250°-1300° C. for homogenization and fining. The glassmelt is then cooled to the temperature corresponding to the viscositysuitable for molding and casting in the form of a bar.

The total duration of the operation is on the order of 2-7 hours. Aftermolding, the glass is annealed at approximately 600°-650° C. with a rateof cooling of 60° C./hour. The properties of the glass are thendetermined as described below.

MEASUREMENTS OF THE PHYSICAL AND CHEMICAL PROPERTIES OF THE GLASS

The measurements of index of refraction and Abbe number are carried outaccording to the usual methods (for n_(d), the yellow line of Hc isused) on annealed samples. The density is measured with the aid of aMicrometrics helium pycnometer.

The integrated transmission of the glass from 380-800 nm is determinedon a polished sample of 10 mm thickness with the aid of a Perkin-ElmerLambda 9 spectrophotometer.

The durability in acid medium is evaluated by the test of the standardDIN 12116. It consists of determining the weight loss of a polishedsample, immersed for 6 hours in boiling 6N hydrochloric acid. The weightloss is expressed in mg/dm².

    __________________________________________________________________________            Example                                                                       (weight %)                                                                    1    2    3    4    5    6    7    8   9   10                         __________________________________________________________________________    La.sub.2 O.sub.3                                                                      23.1 24.1 21.2 21.8 22.6 24.2 22.8 22.6                                                                              22.6                                                                              22.6                       Nb.sub.2 O.sub.5                                                                      23.5 21.2 23.4 23.7 23.2 21.3 23.4 23.2                                                                              23.2                                                                              23.2                       TiO.sub.2                                                                              7.4  8.6  7.5  7.5  7.5  8.6  7.5 7.5 7.5 7.5                        ZrO.sub.2                                                                              8.3  8.3  9.2  8.3  9.1  8.2  9.2 9.1 9.1 9.1                        B.sub.2 O.sub.3                                                                       17.2 16.7 17.5 17.5 18.0 16.3 17.7 18.0                                                                              18.0                                                                              18.0                       SiO.sub.2                                                                              5.6  7.2  6.1  6.1  6.0  7.2  6.1 6.1 6.1 6.1                        CaO     14.9 14.0 15.1 15.1 13.5 13.4 12.9 8.5 8.5 8.5                        Li.sub.2 O                       --    0.4 --  --  --                         Na.sub.2 O                        0.8 --   --  --  --                         SrO                              --   --   5.0 --  --                         BaO                              --   --   --  5.0 --                         ZnO                              --   --   --  --  5.0                        n.sub.d   1.889                                                                              1.885                                                                              1.884                                                                              1.884                                                                              1.883                                                                              1.881                                                                              1.885                                                                             1.883                                                                             1.886                                                                             1.889                     Abbe No.                                                                              30.5 30.6 30.5 30.6 30.6 30.4 30.1 29.9                                                                              29.6                                                                              29.6                       Density gkm.sup.3                                                                      4.01                                                                               4.00                                                                               3.96                                                                               3.97                                                                               3.99                                                                               3.98                                                                               3.98                                                                               4.04                                                                              4.04                                                                              4.05                      T% (10 mm)                                                                            80.8 80.4 80.4 80.4 80.7 81.0 --   --  --  --                         380-800 nm                                                                    Acid Durability                                                                       3421 2904 2918 3410 2785                                              (mg/dm.sup.2)                                                                 __________________________________________________________________________

We claim:
 1. Glasses characterized by the fact that they exhibit anindex of refraction higher than 1.880, an Abbe number of at least 29, adensity lower than 4.1 g/cm³, a good chemical durability and a lowertendency towards crystallization, and an integrated transmission from380-800 nm--at a thickness of 10 mm--higher than 79%, these glassesconsisting essentially, expressed in weight percent on the oxide basis,of

    ______________________________________                                        SiO.sub.2            5-8                                                      B.sub.2 O.sub.3      15-21                                                    ZrO.sub.2             3-10                                                    TiO.sub.2             7-17                                                    Nb.sub.2 O.sub.5     20.5-26                                                  ZrO.sub.2 + TiO.sub.2 + Nb.sub.2 O.sub.5                                                           29-45                                                    La.sub.2 O.sub.3     19-32                                                    Y.sub.2 O.sub.3      0-9                                                      CaO                   8-16                                                    SrO                  0-5                                                      BaO                  0-5                                                      MgO                  0-5                                                      CaO + SrO + BaO + MgO                                                                               8-16                                                    ZnO                  0-5                                                      Li.sub.2 O           0-2                                                      Na.sub.2 O           0-2                                                      K.sub.2 O            0-2                                                      Li.sub.2 O + Na.sub.2 O + K.sub.2 O                                                                 0-2.                                                    ______________________________________                                    


2. Glass according to claim 1, characterized by the fact that itconsists essentially, expressed in terms of weight percent on the oxidebasis, of

    ______________________________________                                        SiO.sub.2            6-8                                                      B.sub.2 O.sub.3      17-18                                                    ZrO.sub.2             6-10                                                    TiO.sub.2             7-11                                                    Nb.sub.2 O.sub.5     20.5-26                                                  ZrO.sub.2 + TiO.sub.2 + Nb.sub.2 O.sub.5                                                           34-41                                                    La.sub.2 O.sub.3     19-26                                                    Y.sub.2 O.sub.3      0-8                                                      CaO                  12-16                                                    SrO                  0-5                                                      BaO                  0-5                                                      MgO                  0-5                                                      CaO + SrO + BaO + MgO                                                                              12-16                                                    ZnO                  0-5                                                      Li.sub.2 O           0-2                                                      Na.sub.2 O           0-2                                                      K.sub.2 O            0-2                                                      Li.sub.2 O + Na.sub.2 O + K.sub.2 O                                                                 0-2.                                                    ______________________________________                                    


3. Glasses according to claim 1, characterized by the fact that theyalso contain up to 0.5 wt % As₂ O₃.
 4. Glasses according to claim 1,characterized by the fact that they contain a maximum of 10 ppm of Fe₂O₃.